GB2026363A - Process for shaping metal alloy products - Google Patents

Abstract

A semi-solid metal alloy charge is shaped under pressure in a die (1, 2, 3, 4). The metal alloy contains discrete degenerate dendritic primary solid particles, in a concentration from 30 to 90% by volume based upon the volume of the alloy, suspended homogeneously in a secondary liquid phase. The process is characterized by low pressures of 25 to 5000 psig and very rapid shaping and solidification times (maximum of 1 m). The process produces complex, close tolerance, high quality metal alloy parts, eg automotive wheels. <IMAGE>

Description

SPECIFICATION Process for shaping metal alloy products This invention relates to a process of forming a shaped metal alloy product and more particularly to a process for producing complex, close tolerance metal alloy parts by press forging.

Shaped metal alloy parts are produced from wrought alloys by forging techniques to obtain optimum physical properties. Where the part has a relatively complex shape, it must normally be formed by utilizing casting alloys, usually at the sacrifice of physical properties. It would be desirable to utilize alloys providing the characteristics of wrought products in a forming process capable of producing complex shapes.

There have recently been developed certain alloys having a microstructure such that they may be cast from a liquid-solid mixture rather than a liquid and thus solidified from a lower temperature than conventional castang alloys. Such alloys and their preparation are disclosed, for example, in U.S. patent 3,948,650 and U.S. patent 3,954,455. As disclosed therein, partially solidified metal alloys, in the form of slurries, can be shaped into alloy parts by a variety of metal forming processes, including die casting, permanent mould casting, closed die forging, hot pressing and other known techniques.

According to the present invention there is provided a process for shaping a metal alloy in which a semi-solid metal alloy charge is shaped in a closed die cavity, said metal alloy containing discrete degenerate dendritic primary solid particles in a concentration of 30 to 90% by volume based upon the volume of said alloy, said primary solid particles being derived from the alloy and being suspended homogeneously in a secondary liquid phase, said secondary liquid phase being derived from the alloy and having a lower melting point than said primary solid particles, said process comprising shaping said metal alloy charge under pressure in said die cavity in a time of less than one second, said die cavity having been preheated to a temperature of between 100 and 450"C., and solidifying the liquid phase of said shaped alloy in the die cavity at a pressure between 25 and 5000 psig in a time of less than one minute.

Embodiments of the invention will now be described with reference of the accompanying drawings in which: Figure 1 is a vertical cross-sectional view of dies in a closed position in a press suitable for use in the invention; Figure 2 is an elevational view of an automobile wheel produced in the press of Fig. 1, and Figure 3 is a plan view of the wheel shown in Fig. 2.

The metal charge or preform used in the process of the invention is semi-solid-a part liquid and part solid mixture. The solid particles, between 30% and 90% of the total volume, are rounded in shape and are normally between about 20 and 200 microns in diameter. This is the result of a prior treatment of the metal in which the metal is melted and then during freezing, is vigorously stirred. This breaks up the grain formation into the generally rounded particles. The resulting metal composition is characterized by discrete degenerate dendritic primary solid particles suspended homogeneously in a secondary phase having a lower melting point than the primary particles. Both the primary and secondary phases are derived from the metal alloy which has been vigorously agitated during freezing.The process and the resulting alloy are more fully disclosed in the aforesaid U.S. patents 3,948,650 and 3,954,455, reference to which should be made for a more complete description thereof.

The generally rounded nature of the discrete degenerate dendritic particles permits the solid particles to flow in a viscous fashion in a continuous liquid matrix. This permits relatively low pressure forming of a part. The pressures used in the process range from about 25 to 5000 psig which permits the forming parts as large as a full sized (14") automobile wheel to be formed in a 250 ton press as compared to a 1 200 ton die casting machine or an 8000 ton press used for convntional forging.

The largely solid nature of the charge, which ranges from 30 to 90%, but preferably over 70%, by volume solids, permits very rapid solidification with a minimum of liquid/solid shrinkage. This, in turn, permits the forming of parts without large "feeding reservoirs" or risers and allows very short residence in the dies. The latter point is vital to the high production rates attainable with this process, e.g. a realistic rate of 240 automobile wheels an hour or 500 small parts an hour may readily be sustained.

The rapid solidification means that nearly all sections of the part, of equal section thickness, will solidify a the same time and thus may be ejected very rapidly, and usually in less than 4 seconds after forming for high conductivity alloys such as aluminium and copper. For ferrous alloys or for parts of relatively large cross-section, solidification time may extend to 1 5 to 20 seconds, but in any event, will always be less than a minute and usually substantially less. The rapid ejection releases the part from many of the constraints of the solid state contraction which normally occurs with decreasing temperaure. Such contraction can progress to the point at which binding on the dies causes high stresses and resulting hot tears or cracks in the shaped part.

Products produced in accordance with the invention possess many of the properties of a forging, but may contain the complex shapes and shape tolerances typical of a casting. The products may be produced using nominally wrought composition alloys having the levels of tensile strength, fatigue strength, ductility and corrosion resistance comparable to forged or wrought products produced from these alloys. Moreover, the process is capable of producing relatively large parts. Automobile wheels, for example, have been prepared having many of the characteristics of forged wheels, utilizing considerably simplified pressing equipment in a considerably more efficient manner than conventionally forged wheels.

In the process of the invention, a preform is heated until 10-70% of its volume becomes liquid. As indicated above, the preform or charge has previously been produced by vigorous agitation of a liquid-solid mixture of the selected alloy which was then rapidly cooled. The temperature to which the preform is heated is between the liquids and solidus temperature for the particular alloy and will vary from heat to heat within a given alloy system depending on the particular chemistry. Since there is no specific temperature at which the metal will form properly, the viscosity as measured by the resistance to penetration of a probe into the semisolid, may be used as an indicator of the % liquid present in the mixture.Generally the range of 5 psig to 1 5 psig will be used, the exact pressure being selected to suit the conditions of the part to be formed. It is possible to avoid cooling and reheating of the preform by using as the charge the vigorously agitated slurry directly-i.e. before it is cooled to form a billet or preform.

Low pressures may be used to shape the preheated billet providing no significant additional solidification occurs during the shaping step. Thus, in order to ensure the use of low pressures, a shaping time in the die cavity of less than one second is required. The die cavity is preheated to a temperature of from 100 to 450"C., depending primarily upon part configuration, in order to prevent significant solidification during the forming or shaping step. If temperatures are too high, there is a tendency for adhesion of the preform to the die, known as die soldering, to occur. During the forming stroke, the pressure raises from zero to the pressure used for solidification. By the end of the forming stroke, the pressure has accordingly risen from about 25 to 5000 psig, usually 500 to 2500 psig, and solidification of the liquid phase begins.Thus, the pressure gradually rises during the shaping stroke and remains at a peak of from 25 to 5000 psig during solidification. The applied pressure enhances heat transfer from the metal alloy to the die and feeds solidification shrinkage. If the pressure is too low, porosity may be at an unacceptable level or complex moulds may fill incompletely. Pressures above 5000 psig may be used, but they are not necessary. Moreover, higher pressures may create a venting problem.

It is desirable to form the part at as low a pressure as possible for reasons of process economy, simplicity of pressing equipment and for die life. Residence time in the die cavity, subsequent to the shaping step, should be short enough, under one minute and preferably less than 4 seconds, to avoid hot cracking of the shaped part from thermal contraction stresses but long enough to complete solidification of the liquid phase of the alloy. Specific times will depend on part thickness. The tendency for hot cracking to occur is a function of alloy composition, fraction solids percent, die tempearture and part configuration. Within the ranges of forming and solidification times herein set forth, times should, of course, be kept as short as possible to maximize part-making productivity.As is apparent from the foregoing discussion, times, pressures, temperatures and alloy solid fraction are a combination of critical variables which together function to achieve the significant process economies and product improvements herein set forth.

The shaping process of the invention may be carried out, for example in a 150-250 ton hydraulic press equipped with dies or moulds of the type illustrated in Fig. 1 of the drawing.

The specific die set there shown is contoured to produce a relatively large complex shape, in this case a highly styled automobile wheel. The die set comprises a movable top die or ram 1, two side dies 2 and 3 and bottom die 4. The dies are shown in closed position, the alloy metal 5 having been shaped into the contour of an automobile wheel.

Another feature of the invention involves the maner in which the dies are vented. The length and diameter of venting channels must be of adequate size to provide ample venting. On the other hand, the channels must normally be sufficiently narrow and long to avoid spraying the molten metal to the exterior of the dies. Venting channels of conventional size, of a diameter used for example in die casting, have proven too narrow to eliminate air pockets in the present press forming process. It has been found, however, that the high solids fraction present during the pressing cycle of the present invention permits wider and shorter venting channels to be used. The result is not only the absence of air pockets in the shaped product, but fewer limitations on die design, the latter because less area is needed to achieve adequate venting.

Four such vents, 6, 7, 8 and 9, are shown in cross-section in Fig. 1. It will be seen from Fig. 1 thet the shaping operation actually involves a concurrent forward extrusion of semi-solid metal into the narrow channels opening into vents 6 and 7, a backward extrusion of semi-solid metal into the channels leading to vents 8 and 9 and a forging stroke against the central portion of the metal in the press. Reference herein to "complex" shapes is intended to identify parts which require such concurrent forward and backward extrusion combined with a forging step in the process herein set forth.

The following example is illustrative of the practice of the invention. Unless otherwise indicated, all parts are by weight.

Example An 18 pound billet of 6061 wrought aluminium alloy was cast, substantially as set forth in U.S. patent 3,948,650, from a semi-solid slurry containing approximately 50% by volume degenerate dendrites. The billet, approximately six inches in diameter, had the following composition: Si Cr Mn Fe Mg Ti Cu B Al 0.63 0.06 0.06 0.22 0,90 0.012 0.24 0.002 Balance The billet, contained in a stainless steel canister, was placed within a resistance furnace set at a temperature of 677"C. This temperature, approximately 28"C, above the liquidus temperature of the alloy, was sufficient to induce partial melting of the alloy without creating significant variations in fraction liquid within the billet.At a temperature of 632"C., corresponding to a fraction solid of approximately 0.80, as detected by the penetration of a weighted probe, the billet in its canister was transferred to the closed bottom half of a cast iron die set, of the type shown in Fig. 1, maintained at 315"C. and ejected from the canister to the bottom of the die.

The die set was coated with a graphite based lubricant. The top die, also maintained with a surface temperature of approximately 315"C., was then closed at a speed of 20 inches per second, resulting in a preform shaping time of about 0.2 seconds, the die reaching a maximum pressure of 2100 psig such that the cavity so formed was filled with alloy. After a holding time under pressure of 2.4 seconds, during which the liquid phase of the part solidified, the die set was opened and the shaped part extracted.

The shaped part, an aluminium wheel, was sectioned and specimens for mechanical property measurement were taken. Room temperature properties were measured. Ultimate tensile strength was 47,000 psi, yield strength was 43,000 psi and elongation in a 1" gauge length was 7%. Minimum specifications for closed die forgings of 6061 aluminium alloys as set forth in Aluminium Standards and Data 1976, Fifth Edition, 1 976 are 38,000 psi ultimate tensile strength, 35,000 psi yield strength and 7% elongation. Representative minimum specifications of an automobile manufacturer for cast aluminium wheels are 31,000 ultimate tensile strength, 16,500 yield strength and 7% elongation.

Unlike wrought products whose properties are directional, the products of the process of the invention are isotropic-their properties are equal in all directions. The metallurgical structure of the wheel of the example consisted of randomly oriented, equiaxed grain structure without the "texture" associated with wrought components having similar properties.

A finished wheel generally identified by the numeral 10 produced in accordance with the invention is shown in elevation in Figs. 2 and 3. The plan view of Fig. 3 shows the wheel as viewed from the direction of the bottom die in Fig. 1. The wheel contains a plurality of roughly rectangular contours 11 around the periphery, each of the contours containing a punched or machined hole 1 2 therethrough. A hub area 1 3 contains four cored and tapped holes 1 4 and four larger punched or machined holes 1 5. A wheel configuration of this complexity is normally produced by permanent mould or die casting techniques and is accordingly limited in its properties to the relatively inferior properties of cast alloys. Material properties are thus a limiting factor on wheel weight.Lower properties must be compensated by greater bulk in a cast wheel. Moreover, larger cross-sections are normally necessary in casting because of limitations inherent in casting techniques-it is difficult to fill a permanent mould with thin sections. Thus, the wheels made by the process of the invention have the very important capability of being lighter in weight than comparable wheels of the prior art.

Representative alloys useful in the press forging process are, in addition to aluminium alloys, ferrous alloys such as the stainless steels, tool steels, low alloy steels and irons and copper alloys of the type normally used in castings and forgings.

It will be recognised that, within the scope of the process parameters set forth herein, many variations may be made in order to accommodate the geometry or the specific property objectives of the component being formed. Changes in alloy chemistry, temperature, speed and pressure of the press and duration of dwell may influence grain structure, avoid shrinkage defects and provide properties to specific portions of the component. Moreover, the process may be used for producing a variety of shaped metal parts other than wheels including, for example, hand tools, valve and pump bodies and parts, propellers and impellers, automotive and appliance parts and electrical and marine components.

Claims (11)

1. A process for shaping a metal alloy in which a semi-solid metal alloy charge is shaped in a closed die cavity, said metal alloy containing discrete degenerate dendritic primary solid particles in a concentration of 30 to 90% by volume based upon the volume of said alloy, said primary solid particles being derived from the alloy and being suspended homogeneously in a secondary liquid phase, said secondary liquid phase being derived from the alloy and having a lower melting point than said primary solid particles, said process comprising shaping said metal alloy charge under pressure in said die cavity in a time of less than one second, said die cavity having been preheated ta a temperature of between 100 and 450"C., and solidifying the liquid phase of said shaped alloy in the die cavity at a pressure between 25 and 5000 psig in a time of less than one minute.

2. A process as claimed in claim 1 in which said metal alloy is solidified at a pressure between 500 and 2500 psig.

3. A process as claimed in claim 1 in which the die cavity is maintained at a temperature of between 200 and 300"C.

4. A process as claimed in claim 1 in which the metal alloy is shaped under pressure in the die cavity in a time between 0.1 and 0.5 seconds.

5. A process as claimed in claim 1 in which the solidification of the liquid phase of the shaped alloy under pressure in the die- cavity occurs in a time of less than 4 seconds.

6. A process as claimed in claim 1 in which the alloy is an aluminium alloy.

7. A process as claimed in claim 1 in which the alloy is a copper alloy.

8. A process as claimed in claim 1 in which the alloy is a ferrous alloy.

9. A process as claimed in claim 1 in which the concentration of the discrete degenerate dendritic primary solid particles is 70 to 90% by volume based upon the volume of said alloy.

10. A process as claimed in claim 1 in which said die cavity is vented to the atmosphere through a plurality of spaced channels extending from the die cavity to atmosphere, said channels being of a size sufficient to exhaust any air entrapped in the die cavity during shaping under pressure.

11. A process for shaping a metal alloy substantially as herein described with reference to the accompanying drawings.